1. Field of the Invention
The invention relates to an electronic package assembly of the type having an encapsulated electronic device, and also to methods for assemblying such packages. More particularly, the invention relates to the securement of pre-plated electrical lead pins to the ceramic substrate of such an electronic package assembly.
2. Description of the Prior Art
It is well known to construct electronic package assemblies from a ceramic substrate having a central cavity in which a semiconductor chip is mounted. The substrate contains an array of holes with each hole receiving an electrical lead pin. The lead pins are secured in the package and a conductive pattern of leads printed on the surface of the substrate electrically interconnects the pins to the semiconductor device. The semiconductor device is often attached in the cavity by means of a gold glass frit paste, commonly referred to as a die attach pad, following which the package is sealed by application of a metal cap enclosing the central cavity to protect the chip against conductive air borne contaminates and other adverse environmental conditions which affect the operating characteristics of the device. Such chip attachment and encapsulation are usually one of the final steps in the package manufacture, taking place after electrical lead pins are secured in the package.
Heretofore, lead pins formed from "Kovar" or similar alloy, were bonded to the ceramic substrate by a copper-silver eutectic braze. The use of copper-silver as a filler alloy between the pins and the ceramic substrate necessitated the brazing operation to be conducted at 780.degree.C. After the pins were secured to the substrate, the lead pattern and pins were plated with a corrosion resistant material, such as gold.
To facilitate the fabrication of such electronic package assemblies and to reduce the cost factor in the prior modes of assembly, it became desirable to eliminate the subsequent step of gold plating the package, by gold plating the lead pins prior to securing the same to the package. However, the use of pre-plated lead pins requires a brazing operation under 600.degree.C. to prevent the gold plating from diffusing into the Kovar-composed pins. But with pin-to-ceramic bonds formed at lower temperatures, problems arise due to the high temperature requisite for subsequent chip attachment within the central cavity. Such high temperatures will degrade the strength and reliability of these low-temperature-formed bonds and could possibly melt such bonds, releasing many of the pins before the chip is attached.
The discovery of a method to braze pre-goldplated pins within a ceramic substrate of an electronic package while still maintaining the integrity of the lead pin bonds during chip attachment has been the subject of continued search in the electronic device art.